Beraprost is a synthetic benzoprostacyclin analogue of natural prostacyclin consisting of four isomers (Beraprost-314d, Beraprost-314d's enantiomer, Beraprost-315d and Beraprost-315d's enantiomer), as shown in Scheme A. Among them, optically pure Beraprost -314d (termed Esuberaprost, APS-314d or BPS-314d) is the pharmacologically active isomer, and is currently under clinical trials as an added-on active pharmaceutical ingredient in the inhaled treprostinil (Tyvaso®) for the treatment of diseases such as pulmonary arterial hypertension in North America and Europe.

The anhydrous form of Beraprost-314d has been previously described, e.g., in Heterocycles, 2000, 53, 1085-1092, U.S. Pat. No. 8,779,170, and WO 2017/174439. According to the chemical structure feature of Beraprost-314d, an anhydrous Beraprost-314d molecule contains free active carbonyl acid functional groups, which can be easily esterified with hydroxyl functional groups of other anhydrous Beraprost-314d molecule to form a dimer at room temperature, and/or with alcohols contained in the residual solvents or excipients to form unwanted ester impurities in storage. Therefore, an anhydrous Beraprost-314d is not stable at room temperature and is unsuitable for storage, shipping, and formulation purposes at commercial considerations.
Beraprost-314d is of great importance from a pharmacologically point of view. Consequently, there is a demand for an efficient and economical preparation of a stable solid form of Beraprost-314d which presents advantages in storage, shipping, handling, and formulation.